CHEMICAL ENERGETICS Enthalpy change, H:- heat change of chemical reaction under const pressure (kJ/mol)
Standard conditions: temp: 25°C, pressure: 1 atm or 101325 Pa
Standard enthalpy change, H:- enthalpy change measured under standard conditions
Endothermic reaction
-heat absorbed > H +ve
-heat content of products higher than reactants (heat absorbed)

Ea: activation energy H: enthalpy change

Exothermic reaction
-heat given off > H -ve
-heat content of products lower than reactants (heat released)

Ea: activation energy H: enthalpy change

Standard enthalpy change of reaction, H_r : heat change when molar quantities of reactants(specified by balance equation) react to for products under standard conditions
Standard enthalpy change of formation, H_f : heat change when 1 mole of product formed from elements in the most stable state under standard conditions
Standard enthalpy change of combustion, H_c: heat change (exothermic only) when 1 mole of element / cpd is completely burnt in excess oxygen under standard conditions

Hess's Law (conservation of heat energy)
-states that total enthalpy change of a chemical reaction is the same regardless of intermediate steps provided initial and final conditions are constant
A + B >(H₁)> C >(H₂)> D >(H₃)> E
A + B >(H₄)> E
By Hess's Law: H₁ + H₂ + H₃ = H₄

Determination of enthalpy change of reaction
Heat evolved = mass × specific heat capacity × temp = mcT = VcT (T = initial - final temp)
enthalpy change of reaction = heat evolved / mol of substance M = VcT/n kJ/mol (of M)
assumptions;
(1) -density of solution = density of water = 1g/cm³
(2) -c_solution = c_water = 4.2J/(g°C)

standard enthalpy change of neutralization, H_n: heat evolved when 1 mole of H⁺(aq) from acid reacts with 1 mole of OH^-(aq) from base to form 1 mole of water under standard conditions of 1 atmosphere & 25°C
assumptions; (1)dilute sol so that
   -density of solution = density of water = 1g/cm³
   -c_solution = c_water = 4.2J/(g°C)
(2)heat loss by conduction is negligible
standard enthalpy change of neutralisation with:
strong acid & strong base
-always constant [-57.3kJ/mol (of H₂O formed)] as molecules are 100% dissociated to ions > heat change only from formation of H₂O molecules (no energy used to dissociate molecules)
weak acid / weak base
-less than -57.3kJ/mol (of water formed) as molecules not fully ionised > heat energy absorbed to dissociate molecules

Born-Haber cycle for formation of ionic solid cpd

H_sub, enthalpy change of sublimation of element: heat absorbed to convert solid element into 1 mole of gaseous atom
M(s) >(H_sub)> M(g)

[M(s) >(H_sub)> M(g) = M(s) >(H_fusion)> M(l) >(H_vap)> M(g)
H_sub = H_fusion + H_vap = enthalphy change of (fusion of solid M+ vaporization liquid M)
H_sub can be also called enthalpy change of atomization,H_atomisation: energy absorbed to change a subs from solid, liquid or gaseous state into 1 mole of gaseous atoms; M(s/l/g) > M(g)
for gaseous diatomic molecules, X₂: H_atomisation = ½ bond energy]

H_IE, 1st ionisation energy: min energy absorbed to remove 1 mole of most loosely bound valence electrons in ground state from 1 mole of isolated, gaseous atoms to form 1 mole of isolated gaseous singly-charged positive ions
   M(g) - e^- >(H_IE)> M⁺(g)
Electron affinity, H_EA: energy evolved when 1 mole of valence electrons is added to 1 mole of isolated gaseous atoms to from 1 mole of isolated, gaseous singly-charge negative ions
X(g) + e^- > X^-(g)
1^st EA- form univalent anion from neutral atom: exothermic (valence electrons added to form anion)
2^nd EA- form divalent anion from univalent anion : endothermic (valence electrons added to anion > energy absorbed to overcome repulsion)
Lattice energy, H_L: energy evolved when isolated, gaseous anions & cations combine to form 1 mole of ionic solid
   M⁺(g) + X^-(g) >(H_L)> M⁺X^-(s)
Factors affecting lattice energy
Charges of ions: greater # of charges > stronger electrostatic force of attraction > greater LE
Ionic radii of ions: smaller radius > stronger electrostatic force of attraction > greater LE

Enthalpy changes of dissolution of ionic solid

Enthalpy change of solution, H_solution: energy change when 1 mole of ionic solid dissolved in such an amt of water that further dilution causes no more heat change (solution of infinite dilution)
M⁺X^-(s) + aq >(H_solution)> M⁺(aq) + X^-(aq)
Enthalpy change of hydration, H_hyd: energy evolve when 1 mole of gaseous ions is hydrated in water to form aqueous ions
M⁺(g) + aq >(H_hyd)> M⁺(aq)


Factors affecting enthalpy change of hydration
Charges of ions: greater # of charges > stronger electrostatic force of attraction > greater HE
Ionic radii of ions: smaller radius > stronger electrostatic force of attraction > greater HE

Bond energy, H_BE (bond enthalpy / bond dissociation energy)
(1)for a diatomic molecule:- energy absorbed to dissociate 1 mole of the X-Y covalent bonds of gaseous XY to form gaseous X & Y atoms
   XY(g) >(H_BE)> X(g) + Y(g)
(2)for a polyatomic molecule:- average energy absorbed to dissociate 1 mole of the X-Y covalent bonds in gaseous XY_n to form gaseous X & Y atoms
   XY_n(g) >(H₁)> XY_n-1(g) + Y(g)
   XY_n-1(g) >(H₂)> XY_n-2(g) + Y(g)
   ........
   XY(g) >(H_n)> X(g) + Y(g)
   bond energy = (H₁ + H₂ + ... + H_n)/n
H_BE increases w/:
-decrease in bond length
-polarity of bond (more polar bond > greater H_BE)

The more exothermic the reaction > the more readily it occurs


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